1. Field of the Invention
The present invention relates to a video random access memory device, and particularly, to a dual port video random access memory device having a block write function.
2. Description of the Related Art
A video random access memory VRAM is a kind of image exclusive memory that operates as a dual port by adding a serial access memory SAM section to a normal dynamic random access memory DRAM.
This device comprises a RAM section capable of randomly reading and writing data and a SAM section capable of serially data accessing across data transmission paths existing between the two memory sections.
The SAM section is formed of data registers storing data as in a static random access memory SRAM cell and has the characteristic of fast access time.
The dual port random access memory device has been developed to provide increased access capability to memory content in a data processing device, and high speed data output to a video display.
The dual port memory operates by having a first port for random access and update of memory under the control of a computer system data processing device and a second port, which is non-synchronous and independent from the first port, for serial output of memory content to a video display so that memory access takes place during data output to the video display terminal.
The dual port random access memory device disclosed in U.S. Pat. No. 4,807,189 requires that the 2 least significant bits of a column address be replaced by a 4 bit data input during a block write function.
As a result, the dual port memory must selectively multiplex either the 2 LSB column address or the 4 bit data input to execute normal DRAM and block write functions.